This invention relates to apparatus and methods for polishing semiconductor wafers or similar type materials, and more specifically to such apparatus and methods which facilitate polishing of a semiconductor wafer to have a flat surface.
Polishing an article to produce a surface which is flat, highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter “lithography”). Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 0.13 microns (5.1 microinches) or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed.
Flatness of the wafer surface can be quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the “focal” plane. SFQR is a specific type of STIR measurement, as measured from the front side best fit reference plane. A more detailed discussion of the characterization of wafer flatness can be found in F. Shimura, Semiconductor Silicon Crystal Technology 191, 195 (Academic Press 1989). Presently, flatness parameters of the polish surfaces of single side polished wafers are typically acceptable when a new polishing pad is being used, but the flatness parameters become unacceptable as the polishing pad wears, as described below.
The construction and operation of conventional polishing machines contribute to unacceptable flatness measurements. Polishing machines typically include a circular or annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the wafer. This type of polishing operation is typically referred to as chemical-mechanical polishing or simply CMP.
During operation, the pad is rotated and the wafer is brought into contact with the pad using the pressure plate. The pressure plate applies a generally uniform downward force across the wafer pressing the wafer against the pad. As the pad rotates, the wafer is rotated and oscillated back and forth about a portion of the pad that is off-center. As a result, pad wear is most significant in an annular band AB, which is illustrated in FIG. 1 by dark shading, that is contacted by the wafer during every revolution of the pad. The pad wear is gradationally less severe in the areas LA extending away from the annular band AB. These areas are only contacted by the wafer during some of the revolutions of the pad. Moreover, the portions of the pad farther from the annular band are contacted less frequently than portions of the pad closer to the annular band. As a result, these areas LA, which are represented in FIG. 1 by shading that becomes gradationally lighter away from the annular band, experience gradationally pad wear that is less severe away from the annular band and more severe closest to it. The outer most OM and inner most IM portions of the pad do not contact the wafer during the polishing operation and therefore do not experience any significant wear. These areas OM, IM are free from shading in FIG. 1.
When the pad wears, e.g., after a few hundred wafers, wafer flatness degrades because the pad is no longer flat but instead has an annular depression corresponding to the annular band AB of FIG. 1. Typically, such pad wear impacts wafer flatness in one of two ways: “dishing” and “doming”. “Doming”, which is more common than “dishing” and illustrated in FIG. 2, results in the wafer having a generally convex front surface (the front surface of the wafer is the surface polished by the pad). This results when the pad is worn as illustrated in FIG. 1 and, as a result, removes less material from the center of the front surface of the wafer than from the areas closer to the wafer's edge. This is because the pad's removal rate is inverse to its wear. In other words, the portions of the pad with less wear remove more material than portions of the pad with more wear. The least amount of material is removed from the wafer by the portion of the pad corresponding to the annular band AB. As a result, the front surface of the wafer is caused to have a generally “domed” shaped.
“Dishing” of the wafer surface occurs when the front surface of the wafer is caused to have a concave upper surface, which is illustrated in FIG. 3. One potential reason for this occurring is that the polishing pad becomes embedded with abrasives (i.e., colloidal material from the slurry, debris from previously polished wafers, debris from a retaining ring) thereby causing the removal rate to increase in the areas of wear. That is, the removal rate of the pad is directly proportional to its wear. Thus, the portions of the pad with more wear remove more material from the wafer during the polishing process than portions of the pad with less wear. As a result, more material is removed from the wafer from the portion of the pad corresponding to the annular band AB illustrated in FIG. 1 than from portions of the pad outward from the annular band. This discrepancy in removal rate causes more material to be removed from the center of the wafer than from its edge resulting in the front surface of the wafer having a generally “dished” shape.
When the flatness of the wafers becomes unacceptable (e.g., too “domed” or too “dished”), the worn polishing pad has to be replaced with a new one. Frequent pad replacement adds significant costs to the operation of the polishing apparatus not only because of the large number of pads that need to be purchased, stored, and disposed of but also because of the substantial amount of down time required to change the polishing pad.
Accordingly, there is a need for a polishing apparatus that inhibits both doming and dishing of the front surface of wafers during the polishing process and extends the useful life of the polishing pad.